Data communication systems typically involve a transmitter, a receiver, and a communication path between the transmitter and receiver. The transmission path may be air or cables (wire or optical fiber). Although the present invention may be used in many different data communication system applications, it is described herein in the context of a cable system. However, it should be understood that the cable system environment is merely an exemplary environment for the present invention and that the present invention may be used in many other environments.
A cable system typically includes a head end which transmits data to a plurality of subscribers over a cable network. Typically, the cable network is at least partially buried and has a main trunk carrying data directly from the head end, branch lines branching out of the main trunk, and subscriber lines carrying data between the branch lines and the subscribers. Considerable labor is required in running the subscriber lines from the branch lines to subscribers, particularly for those subscribers who are located at distances such as 1,000 feet or more from the branch lines.
Instead of running subscriber lines from branch lines to subscribers, transmitters could be located periodically along the branch Lines in order to transmit data over the air between branch lines and subscribers. Thus, the substantial labor which is necessary to connect a subscriber to a branch line is materially reduced. However, care must be exercised in locating such transmitters. For example, if a subscriber is covered by only one transmitter, there may be areas within the premises of the subscriber where reception is poor.
The possibility of poor reception can be lessened by locating the transmitters so that the premises of each subscriber is covered by two or more transmitters. Unfortunately, because each transmitter operates at the same carrier frequency, and because of the variable distances between a subscriber's premises and the transmitters that cover the subscriber's premises, the same data may arrive at a reception site within a subscriber's premises at different times and with different phases. As a result, interference, referred to herein as ghosting, is produced.
If signal amplitude versus frequency of the received signal at a reception site in a subscriber's premises covered by two transmitters is graphed, an interference pattern can result. In the case where the reception site is located at an equal distance from both transmitters, the resulting interference pattern is characterized by periodic, sharply defined nulls at which the received signal is substantially undetectable, particularly in the presence of noise. That is, noise in the channel establishes a signal detection threshold such that any frequency components of the transmitted signal near or at the nulls will be difficult or impossible to detect because the signal to noise ratio at these points is too low. Moreover, when the received signal is processed through an equalizer, the signal to noise ratio can worsen, making signal detection even more difficult.
It is known how to adequately receive signals in the presence of white noise. For example, trellis encoding and Viterbi decoding may be used to encode and decode transmitted data adequately when white noise is present, because this type of coding and decoding performs well under white noise conditions. Unfortunately, trellis encoding and Viterbi decoding do not work particularly well in the presence of non-randomly distributed noise, such as may be present in an environment experiencing the above described interference pattern.
However, as disclosed in co-pending U.S. patent application Ser. No. 09/052,501 field Mar. 31, 1998, data may be transmitted as code vectors which may be decoded in the receiver in such a lessen the effect of non-randomly distributed noise on the recovery of the data from the transmitted signal. When code vectors are used to transmit data, the receiver must be synchronized to the received signal so that the transmitted code vectors can be accurately recovered and decoded. Prior synchronization arrangements are not: useful and/or efficient for the accurate recovery and decoding of transmitted code vectors.
The present invention is arranged to overcome one or more of the above-stated problems.